Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P42: Complex Oxide Interfaces & Heterostructures - Skyrmions and Novel Magnetism |
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Sponsoring Units: GMAG DMP Room: 709/711 |
Wednesday, March 4, 2020 2:30PM - 2:42PM |
P42.00001: Topological Hall Effect in SrTiO3/SrIrO3/SrRuO3 Superlattices Jose Flores, Adam S Ahmed, Binbin Wang, Nuria Bagues Salguero, David W. McComb, Fengyuan Yang Ferromagnet(FM)/heavy-metal(HM) bilayers films have been found to able to host topological magnetic structures like skyrmions stabilized by exchange and interfacial Dzyaloshinskii-Moriya interactions. Electrical measurement by the topological Hall effect (THE) has been a primary method of detection of these features. Perovskites multilayers are an excellent platform for investigating these systems due to the wide range of electrical and magnetic properties and very low lattice mismatch allowing for the growth of high-quality films. Here we investigated the topological hall effect in STO(001)/SrRuO3/SrIrO3/SrTiO3 superlattices where SrTiO3 acts as a spacer layer that seperates the FM/HM layers SrRuO3/SrIrO3. As the spacer thickness was increased, the magnitude of the topological Hall resistivity decreased suggesting interlayer coupling. Superlattices with thinner spacer layers show a transition between two topological hall peaks corresponding to before and after the magnetization has switched and an intermediate temperature where both signal are observed. This behavior appears to be unique to the superlattices and is not seen in bilayer structures. |
Wednesday, March 4, 2020 2:42PM - 2:54PM |
P42.00002: Tunable real- and momentum-space topological properties in SrRuO3 ultrathin films Lingfei Wang, Tae Won Noh In perovskite-structured SrRuO3 (SRO), a fine balance between the electron-electron correlation and spin-orbit coupling gives rise to a variety of exotic topological properties. Here, we will show our recent results about tunable real-and momentum-space topological properties in SRO ultrathin films. First, we report the discovery of ferroelectrically tunable skyrmions in ultrathin BaTiO3/SRO bilayer heterostructures. The ferroelectric proximity effect at the BaTiO3/SRO heterointerface can trigger a sizable Dzyaloshinskii-Moriya interaction, thus stabilizing a real-space topological spin texture: magnetic skyrmion. By locally manipulating the ferroelectric polarization, we can achieve local, switchable, and nonvolatile control of both skyrmion density and thermodynamic stability. Second, we will focus on the highly tunable anomalous Hall effect (AHE) in SRO single layers. We found the AHE exhibit a clear sign reversal as the SRO film thickness decreases to 4 unit-cells due to the changes in momentum-space Berry curvatures. By harnessing the step-flow growth, we can induce an artificial thickness inhomogeneity and a two-channel AHE in SRO films. We developed several experimental methods for distinguishing the skyrmion-induced topological Hall effect from the two-channel AHE. |
Wednesday, March 4, 2020 2:54PM - 3:06PM |
P42.00003: Investigating the Skyrmion Phase Diagram in Polycrystalline Chemically Substituted Cu2OSeO3 with Small Angle Neutron Scattering Paul Neves, Dustin Gilbert, Sheng Ran, I-Lin Liu, Shanta Saha, John Collini, Markus Bleuel, Johnpierre Paglione, Julie Ann Borchers, Nicholas Butch The chiral helimagnet Cu2OSeO3 is now understood to host a variety of interesting magnetic phases. The stability of these phases is sensitive to many parameters, including magnetic field and temperature history, magnetic field direction, and chemical tuning. Here, we discuss results studying the magnetic phase diagram of chemically substituted polycrystalline (Cu1-xMx)2OSeO3, M = (Ag, Zn), and Cu2(O1-xSx)2SeO2 with bulk magnetization and small angle neutron scattering (SANS). Notably, we perform SANS measurements after rotation of our powdered samples in a magnetic-field to resolve 6-fold symmetric skyrmion lattice diffraction patterns. This allows us to gleam information about the skyrmion lattice order and magnetocrystalline anisotropy. Bulk magnetization suggests an enhancement of skyrmion temperature stability in the Ag and S substituted samples, while SANS suggests a generic decrease in the ability of field rotation to align skyrmion lattices in chemically disordered systems. Additionally, we are unable to confirm the presence of a second skymion phase in Zn substituted samples. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P42.00004: Theoretical investigations of Topological Hall Effect at Manganite/Iridate Oxide Interfaces Narayan Mohanta, Elbio Dagotto, Satoshi Okamoto We study topological Hall effect at the interface between La1−xSrxMnO3 and SrIrO3. We show that the ferromagnetism at the manganite layer, in conjunction with the Dzyaloshinskii-Moriya (DM) interaction which arises at the interface due to broken inversion symmetry and strong spin-orbit coupling from the iridate layer produces a skyrmion crystal (SkX) phase in the presence of an external magnetic field. Using the Monte Carlo technique and a two-orbital spin-fermion model for manganites, we obtain phase diagrams which reveal, at low temperatures, a clear SkX phase and also a low-field spin-spiral phase. Increasing temperature, a skyrmion-gas phase, precursor of the SkX phase upon cooling, was identified. The topological Hall effect primarily appears in the SkX phase. We also show that a planar topological Hall effect can robustly appear at oxide interfaces from conical spin spirals at oxide interfaces, without skyrmions, when the magnetic field is applied parallel to the interface plane. |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P42.00005: Investigation of Domain Walls in Single Crystal Cr2O3 Using Nitrogen Vacancy Scanning Magnetometry Natascha Hedrich, Kai Wagner, Brendan Shields, Tobias Kosub, Denys Makarov, Patrick Maletinsky Magnetoelectric antiferromagnets are promising candidates for spintronic devices that have been used to demonstrate electrical reading and writing [1]. Chromia, in particular, has drawn attention due to its room temperature ordering. While domain structures in thin-film Cr2O3 have been studied in detail [2], domains in single crystals prove more elusive and less well understood. Second harmonic generation and magnetic force microscopy have been used to study these structures, but these techniques have provided primarily qualitative images [3,4]. Here, we present a quantitative study of domain walls in single crystal chromia using nitrogen vacancy (NV) magnetometry, performed with all-diamond scanning probes, where we make use of the sub 1 uT/√Hz sensitivity and sub 50 nm spatial resolution of these structures to investigate the domain wall properties at the nanoscale. Our measurements demonstrate the unique possibility offered by NV magnetometry to study single crystalline antiferromagnetic systems and reveal fundamental properties of the crystal and its spin textures. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P42.00006: Spin Dynamics in the Antiferromagnetic Oxy- and Fluoro- Pnictides: LaMnAsO, LaMnSbO, and BaMnAsF FARHAN ISLAM, Pinaki Das, Yong Liu, Elijah Gordon, Liqin Ke, Douglas L Abernathy, Robert McQueeney, David Vaknin Inelastic neutron scattering (INS) from polycrystalline LaMnAsO, LaMnSbO, and BaMnAsF are analyzed using J1-J2-Jc Heisenberg model in the framework of the linear spin-wave theory. All three systems show clear evidence that J1 and J2 are both antiferromagnetic (AFM) with a ratio of 2J2/J1 < 1. Thus, the competing interactions do not lead to the stripe structure, preserving the square lattice checkerboard AFM structure. The inter-plane coupling Jc in all three systems is on the order of ≈ 3×10-4J1, rendering the magnetic properties of these systems quasi-two-dimensional. The substitution of Sb for As significantly lowers the in-plane exchange coupling, which is also reflected in the Néel temperature TN. Although BaMnAsF shares similar MnAs sheets as LaMnAsO, their J1-J2 values are substantially different. Using density functional theory, we calculate exchange parameters Jij to rationalize the differences between these systems. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P42.00007: Magnetic structure and spin dynamics of a buckled-honeycomb antiferromagnet Co4Ta2O9 Sungkyun Choi, Valery Kiryukhin, Matthias J Gutmann, David J Voneshen, Shangke Pan, Sang-Wook Cheong Controlling magnetism by the electric field is one of the most important topics in correlated materials, which could be realized via magnetoelectric (ME) coupling. The discovery of ME in Cr2O3 [1] initiates intense research on related ME materials for practical applications. However, there are not many real materials that show strong ME coupling due to rather strict symmetrical conditions: the absence of the inversion and time-reversal symmetry. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P42.00008: Direct visualization of domain dynamics in Ni2MnTeO6 antiferromagnet using X-ray Bragg diffraction Phase contrast Imaging. Min Gyu Kim, Sungkyun Choi, Hu Miao, Choongjae Won, Junjie Yang, Wen Hu, Claudio Mazzoli, Sang-Wook Cheong, Andi Barbour, Stuart B Wilkins, Ian Keith Robinson, Mark Dean, Valery Kiryukhin Magnetic phase transitions provide one of the important testbeds in condensed matter physics. Development of the magnetic domains lies at the heart of any such transitions. Generally, antiferromagnetic (AFM) domains can evolve with temperature, through the phase transition from the paramagnetic phase to the ordered state, or between states with different AFM orders. However, the study of such dynamics of AFM domain was impossible due to the lack of temporal resolution in the existing techniques. Using recently developed X-ray Bragg diffraction Phase contrast Imaging (XBPI) technique that can visualize AFM domains and domain walls in real-time and real-space, we present the growth and merging of the AFM domains in real-time as a function of temperature in Ni2MnTeO6 where two AFM states with different Q vectors exist. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P42.00009: Random Telegraph Noise in Ferromagnetic Garnets Robert Sponsel, E. Dan Dahlberg Magnetic noise in ten garnet samples is reported. The magnetic noise, fluctuations in the transmitted light intensity using the Faraday effect to observe the magnetization, was measured when the samples were driven by an applied AC magnetic field. The data were taken as a function of the area sampled, the magnitude of the AC magnetic field, and the AC frequency (40-250 Hz). In zero applied magnetic field, the magnetization of the samples consisted of either stripe or serpentine domains. The measured noise is white at fields somewhat below and somewhat above the coercive field for all length scales. Intermediate fields, close to the coercive field of a sample, exhibit a Lorentzian power spectral density (PSD) as would be expected for random telegraph noise (RTN). This PSD was seen at all length scales. Surprisingly, the Lorentzian noise exhibits the same characteristic frequency at all positions measured in a given sample. With increasing area the magnitude of the Lorentzian noise decreases following a 1/area relation consistent with the Lorentzian noise arising from many uncorrelated RTN oscillators. The results will be discussed in terms of random pinning of domains to create complex domain structures. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P42.00010: Magnetic structure and electronic properties in Layered BaMn2Sb2 and Ba2Mn3Sb2O2 Qiang Zhang, Zhenyu Diao, Huibo Cao, Ahmad Ikhwan Us Saleheen, Ramakanta Chapai, Dongliang Gong, Shane Stadler, Rongying Jin We present the structural, electronic, and magnetic properties of BaMn2Sb2 and Ba2Mn3Sb2O2 single crystals, both with the layered structure analogous to high-temperature Fe- and/or Cu-based superconductors. While the Mn moment in the MnSb4 tetrahedral environment forms a long-range G-type antiferromagnetic (AFM) ordering in both BaMn2Sb2 (TN1 ≈443 K) and Ba2Mn3Sb2O2 (TN1 ≈314 K), a short-range AFM order is found in the intercalated MnO2 layer at a much lower temperature (TN2 ≈60 K) coexisting with long-range G-type order in MnSb4 layer in Ba2Mn3Sb2O2. The directions of the ordered moments in these two magnetic sub-lattices of Ba2Mn3Sb2O2 are perpendicular to each other. The G-type AFM order in MnSb4 layers in both compounds is distinct from the stripe-like order in Fe-based superconductors, but the in-plane magnetic structure in MnO2 layers of Ba2Mn3Sb2O2 is found to be similar to that in the Cuprates. Our study reveals the significant and distinct roles of the CuO2- and FeAs4-type transition metal layers in the structure, novel magnetic and electronic properties. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P42.00011: Calculated magnetic exchange interactions in the Dirac magnon material Cu3TeO6 Di Wang, Xiangyan Bo, Feng Tang, Xiangang Wan Recently, topological aspects of magnon band structure have attracted much interest, and especially, the Dirac magnons in Cu3TeO6 have been observed experimentally. In this work, we calculate the magnetic exchange interactions J's using the first-principles linear-response approach and find that these J's are short range and negligible for the Cu-Cu atomic pair longer than 7 Å. Moreover, there are only five sizable magnetic exchange interactions, and according to their signs and strengths, modest magnetic frustration is expected. Based on the obtained magnetic exchange couplings, we successfully reproduce the experimental spin-wave dispersions. The calculated neutron scattering cross section also agrees very well with the experiments. We also calculate Dzyaloshinskii-Moriya interactions (DMIs) and estimate the canting angle of the magnetic noncollinearity based on the competition between DMIs and J's, which is consistent with the experiment. The small canting angle agrees with that current experiments cannot distinguish the DMI-induced nodal line from a Dirac point in the spin-wave spectrum. Finally, we analytically prove that the “sum rule” conjectured in W. Yao et al. [Nat. Phys. 14, 1011 (2018)] holds but only up to the 11th-nearest neighbor. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P42.00012: Elucidation of spin reorientation and short-range ferromagnetic correlations in double perovskite Ho2FeCoO6 Thomas Heitmann, HARIPRIYA G R, K. Sethupathi, V. Sankaranarayanan, R. Pradheesh, Harikrishnan S Nair We report on energy resolved neutron diffraction and inelastic neutron scattering studies of double perovskite Ho2FeCoO6. The B-site moments have been shown previously to order antiferromagnetically near 250 K with a spin reorientation at 45 K. We use energy resolved neutron diffraction of a Ho2FeCoO6 powder to establish the order parameter of these two phases and to characterize a diffuse scattering signal observed surrounding q = 1.3 Å-1. The diffuse part is determined to result from short-range ferromagnetic correlations of the Ho-moments. The development of Ho correlations coincides in temperature with the spin reorientation transition so it is likely that exchange between sublattices is becoming important in this temperature range. We have also explored this system via inelastic scattering and report on a crystalline electric field that is detected at E = 9.8 meV. Both the diffuse and inelastic signals were found to be absent in non-A-site magnetic La2FeCoO6 confirming the origin of these effects as the Ho sublattice. |
Wednesday, March 4, 2020 4:54PM - 5:06PM |
P42.00013: Precursor effects to the Verwey transition of Fe3O4: a MaxEnt-μSR study. Carlos Morante, Carolus Boekema The magnetic fields of Fe3O4 have been previously studied using muon-spin rotation (μSR). [1,2] The Verwey transition at Tv(~123 K) and a transition at Tw(~247 K) are observed. Using Maximum Entropy (MaxEnt, ME) the single-crystal Fe3O4 μSR data are analyzed with improved precision. [3] Extra signals are found, indicating 2 signals at roomtemperature (RT) and 2 at 205 K. [4] These frequencies are found at Bext // <110> and Bext smaller than the demagnetization B. At RT, the upper signal follows the trend seen in the Tv-Tw region. At 205 K, the lower signal follows the extension above Tw. Short-range ordering could be interpreted as precursor effects to the Tv transition. [1,2] This short-range ordering relates to the “extra” 3d-electron (3d*) behavior. We re-examine the T dependence of fields with B (720 Oe) // <110>. Our Fe3O4 MEμSR study of the 3d* spin-phonon interaction could lead to a better understanding of the magnetism of this Mott-Wigner glass. [1] C Boekema et al, Hpf Interactions 31 (1986) 487; 17-19 (1984) 305. [2] C. Boekema et al, Phys Rev B33 (1986) 2102; Phys Rev B31 (1985) 1233. [3] C Boekema and MC Browne, MaxEnt 2008, AIP Conf Proc #1073 (2008) 260. [4] C Morante and C Boekema, MMM19 Las Vegas (2019) submitted. |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P42.00014: Spin waves above and below the Verwey transition in TbBaFe2O5 Daniel Pajerowski, Daniel K. Pratt, Steven E Hahn, Wei Tian, Garrett E Granroth, Alexander Kolesnikov, Alexey A Taskin, Yoichi Ando, Robert McQueeney TbBaFe2O5+δ (TBFO) is a mixed valence compound with antiferromagnetic order (TN = 450 K) that changes along with the crystal structure and conductivity upon cooling below a Verwey-like transition (TV = 280 K). This type of double-cell-layered-perovskite is useful to study putative charge ordering in fractional valence systems because the crystal structure develops unique sites for the different valence states. While the static magnetic structure of TBFO is known, in this study we investigate the magnetic interactions. Single crystal inelastic neutron scattering experiments at T = 4 K < TV and T = 320 K > TV show spin-waves that are modeled with linear spin wave theory to extract magnetic anisotropy and superexchange parameters. Below TV there is a sizable magnon gap due to an easy-axis magnetic anisotropy that closes above TV resulting from the evolution to easy-plane anisotropy. The magnetic interactions are found to be highly three-dimensional and change across TV. Above TV, where the conductivity is greater, a damping term is required to model the spin waves. |
Wednesday, March 4, 2020 5:18PM - 5:30PM |
P42.00015: Indirect Chiral Magnetic Exchange through Dzyaloshinskii-Moriya–Enhanced RKKY Interactions Matthias Bode, Martin Schmitt, Chong H Park, Paula Weber, Andreas Jäger, Jeannette Kemmer, Matthias Vogt Localized electron spins can couple magnetically via the Ruderman-Kittel-Kasuya-Yosida interaction even if their wave functions lack direct overlap. Theory predicts that spin-orbit scattering leads to a Dzyaloshinskii-Moriya type enhancement of this indirect exchange interaction, giving rise to chiral exchange terms. Here we present a spin-polarized scanning tunneling microscopy study of transition metal oxide chains on the (001) surfaces of Ir and Pt. Our STM results confirm that deposition of Co, Fe, Mn, and Cr on the (2×1) oxygen-reconstructed Ir(001) surface leads to the formation of quasi-one-dimensional chains with a (3×1) unit cell. In contrast, preparation on Pt(001) required deposition onto the cold substrate with subsequent annealing in an oxygen atmosphere. In particular, for MnO2 chains we found highly complex spin structures. Whereas we find an almost antiferromagnetic Mn–Mn coupling along the chains, the inter-chain coupling across the non-magnetic substrates turns out to be chiral. We will show that some of the magnetic structures observed for MnO2 can be viewed as highly anisotropic Skyrmions. |
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